When viewing the Operation History list for a group or resource, we should provide a cancel button to request the operation be canceled. When pressing the cancel button, the UI should call:
org.rhq.enterprise.server.operation.OperationManagerBean.cancelOperationHistory(Subject, int, boolean)

git commit to master:
a96bf77b2e402a29dd2265ad2b2bf80e5459bb88 (the actual code to implement this BZ)
7771076c4c28709021d601b03ad79fcc05eaf5a8 (a minor i18n commit, nothing substantial)
There is still one issue - when you cancel an operation, the status shows as "Failure" with the Failure icon. It should show the Cancel status/icon if we can determine the operation was actually canceled. I'll create a new BZ for that, since as it stands now with these commits, we can cancel.

how to test:
1) create a executable shell script that sleeps before exiting - something like:
#!bin/sh
echo "Sleeping for 60 seconds"
sleep 60
echo "Exiting with exit code 0"
exit 0
2) Import a platform and create a child resource by manually importing that script as a "Script Server" resource (this is the script plugin).
3) The "Script Server" resource has an operation called "execute". Invoke that operation so your test script is invoked by the agent.
4) When you see the operation history item show up as "in progress" in the Operation>History list, select that "in progress" item and click the "Cancel" button.
5) Wait to see that the operation got canceled. By refreshing the history list, you should see its status change to "Canceled".
6) Create a new compatible group and put your Script Server resource in that group
7) Go to that group's Operation tab and invoke the "execute" operation.
8) Go to the group Operation>History subtab and use the Cancel button to cancel the group operation
9) You should eventually see the GROUP operation show "Failure" (the red failure icon goes in the status) meaning the group operation failed (that is, one or more resource operations did not complete successfully).
10) Drill down into that group operation (double click it) and see that the individual resource operation shows the Canceled status.